Route optimization method and agent apparatus

ABSTRACT

A foreign agent to which a correspondent terminal is attached receives a Binding Update Message from a home agent so that the foreign agent encapsulates an IP packet destined for a mobile terminal  11  before transmission thereto.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to a route optimization methodand an agent apparatus and, more particularly, to a route optimizationmethod and an agent apparatus by which an IP packet route is optimizedin a communication system employing the Mobile IP protocol.

2. Description of the Related Art

Mobile IP, proposed by Internet Engineering Task Force (IETF), is knownas a method for a mobile terminal having an IP address to maintaincommunication without terminating a session even when the mobileterminal moves across network domains.

FIG. 30 shows how a packet is transmitted according to the Mobile IPprotocol of the related art.

Referring to FIG. 30, the mobile terminal 11 has a permanent IP addressin a home domain (hereinafter, simply referred to as a home address). Ahome agent (HA) 13 is located in the home domain to which the mobileterminal 11 belongs. Numeral 41 indicates a correspondent terminal withwhich the mobile terminal 11 communicates. The correspondent terminal 41may be a land unit or a mobile unit. An abbreviation CN/MT(correspondent node/mobile terminal) will also be used to denote thecorrespondent terminal 41. Numeral 201 indicates a foreign agent (FA)located in a destination domain of the mobile terminal 11 and has acare-of address.

A description will now be given of how a packet is transmitted accordingto Mobile IP.

After moving to a destination domain, the mobile terminal 11 acquires anaddress assigned to itself in the destination domain in the form of acare-of address. The mobile terminal 11 notifies the home agent 13 ofthe care-of address via the foreign agent 201. The home agent 13registers the care-of address from the mobile terminal 11 in relation tothe home address of the mobile terminal 11.

The correspondent terminal 41, requesting an IP packet to be sent to themobile terminal 11, sends an IP packet destined for the home address ofthe mobile terminal 11. The IP packet arrives at the home agent 13 viaan IP network 220. The home agent 13 encapsulates the IP packet based oninformation registered for the mobile terminal 11 and then forwards theencapsulated packet to the care-of address of the mobile terminal 11.The foreign agent 201 receiving the encapsulated IP packet decapsulatesthe packet and delivers the original IP packet to the mobile terminal 11having the care-of address.

When the mobile terminal 11 is located in the home domain, the homeagent 13 operates as a router so that the IP packet addressed to themobile terminal 11 is routed to the mobile terminal 11.

Thus, even when the mobile terminal 11 moves across domains, the IPpacket addressed to the mobile terminal 11 is ensured to arrive at themobile terminal 11 via the home agent 13 and the foreign agent 201.

One problem with the base Mobile IP protocol is that, when the IP packetaddressed to the mobile terminal 11 arrives at the mobile terminal 11via the home agent 13, a packet may follow a path which is longer thanan optimal path. When this triangular routing occurs, a relatively largetransmission delay may be incurred. The transmission delay presents aserious problem when the data transmission is concerned with voice overIP datagrams, which are relatively less tolerant of delay.

An internet draft from the IETF (draft-ietf-mobileip-optim-08.txt)proposes Route Optimization to resolve the problem of transmission delaycaused by triangular routing. Route Optimization uses a protocol forestablishing a shortcut path from the correspondent terminal to thecare-of address of the mobile terminal 11, bypassing the home agent 13.FIG. 31 shows Route Optimization applied to the related-art Mobile IP.

In the proposed Route Optimization, the correspondent terminal 41acquires the care-of address of the mobile terminal 11 by periodicallysending a Binding Request Message to the home agent 13 for the mobileterminal 11 and receives a Binding Update Message therefrom. Thecorrespondent terminal 41 extracts the current care-of address of themobile terminal 11 from the Binding Update Message periodicallyobtained. The correspondent terminal 41 encapsulates the IP packetdestined for the mobile terminal 11 and transmits the encapsulatedpacket to the care-of address.

Thus, as shown in FIG. 31, the IP packet from the correspondent terminal41 addressed to the mobile terminal 11 arrives at the mobile terminal11, bypassing the home agent 13. This way, the problem with triangularrouting is resolved.

One disadvantage with Route Optimization described above is that thecorrespondent terminal 41 must exchange messages with the home agent 13for the mobile terminal 11 periodically, making it difficult to reducepower consumption in the correspondent terminal 41. This presents aproblem especially when the correspondent terminal 41 is abattery-powered mobile unit.

In the related-art Route Optimization, the size of a header portion ofthe IP packet doubles due to encapsulation, thus preventing thebandwidth for transmission from the correspondent terminal 41 to theforeign agent or the home agent of the domain to which the correspondentterminal 41 belongs from being effectively used. Particularly, VoIPpackets presents a serious problem since the size of a header portion isrelatively large in relation to the data portion.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide a routeoptimization method and an agent apparatus in which the aforementionedproblems are eliminated.

Another and more specific object is to provide a route optimizationmethod and an agent apparatus in which the Binding Update Message fromthe home agent is received by a home agent or a foreign agent to whichthe correspondent terminal 41 is attached and the home agent or theforeign agent encapsulates a packet destined for the mobile terminal 11for transmission thereto, thereby eliminating the need for periodicalmessage exchange between the correspondent terminal 41 and the homeagent 13 for the mobile terminal 11, reducing the power consumption ofthe correspondent terminal 41, and effectively using the transmissionbandwidth by reducing the volume of data transmitted from thecorrespondent terminal 41 to the foreign agent of the domain to whichthe correspondent terminal 41 belongs.

The aforementioned objects can be achieved by a route optimizationmethod for a communication system that allows communication between amobile terminal and a correspondent terminal, for optimizing a route forcommunication between the correspondent terminal and the mobile terminalwhen the mobile terminal moves across network domains, comprising thesteps of: causing a foreign agent or a home agent of a network domain towhich the correspondent terminal currently belongs to receive a BindingUpdate Message from a home agent for the mobile terminal and to forwarda packet destined for the mobile terminal to a care-of address of themobile terminal specified in the Binding Update Message.

The route optimization method may further comprise the steps of: causingthe foreign agent or the home agent of the network domain to which thecorrespondent terminal currently belongs to repeat transmission of aBinding Request Message to the home agent for the mobile terminal inorder to acquire the latest care-of address; and causing the home agentfor the mobile terminal to transmit, in response to the Binding RequestMessage, a Binding Acknowledge Message containing the latest care-ofaddress of the mobile terminal to the foreign agent or the home agent ofthe network domain to which the correspondent terminal currentlybelongs.

The route optimization method may further comprise the steps of: causingthe foreign agent or the home agent of the network domain to which thecorrespondent terminal belongs to combine a plurality of Binding RequestMessages for obtaining the care-of addresses of a plurality of mobileterminals into an extended Binding Request Message and to send theextended Binding Request Message, when the plurality of mobile terminalsare coupled to the same home agent.

The foreign agent or the home agent of the network domain to which thecorrespondent terminal currently belongs may use subnet masking tomaintain a list of home agents capable of interpreting an extendedBinding Request Message, so as to send the extended Binding RequestMessage for the plurality of mobile terminals to the home agent capableof interpretation and send the Binding Request Message for each mobileterminal to the other home agents.

The home agent for the mobile terminal may notify the foreign agent orthe home agent of the network domain to which the correspondent terminalcurrently belongs whether the home agent is capable of interpreting theextended Binding Request Message, and the foreign agent or the homeagent of the network domain to which the correspondent terminalcurrently belongs may dynamically determine whether the home agent forthe mobile terminal is capable of interpreting the extended BindingRequest Message, based on the notification, so that the foreign agent orthe home agent sends the extended Binding Request Message for theplurality of mobile terminals to the home agent capable of interpretingthe extended Binding Request Message and sends the Binding RequestMessage for each mobile terminal to the home agent.

The foreign agent or the home agent of the network domain to which thecorrespondent terminal currently belongs may accept only the BindingUpdate Message from the selected home agents.

The foreign agent or the home agent of the network domain to which thecorrespondent terminal currently belongs may use a subnet mask tomaintain a list of home agents originating the acceptable Binding UpdateMessage.

The foreign agent or the home agent may forward only the packet from theselected correspondent terminals to the mobile terminal.

The foreign agent or the home agent may use a subnet mask to designate agroup of correspondent terminals with respect to route optimization.

The foreign agent or the home agent of the network domain to which thecorrespondent terminal currently belongs may control an interval oftransmission of the Binding Request Message in accordance with afrequency of change of the care-of address.

The foreign agent or the home agent of the network domain to which thecorrespondent terminal currently belongs may set an initial value ofpriority of update for each mobile terminal, compute the priority ofupdate in accordance with the frequency of change of the care-ofaddress, and set the interval of transmission of the Binding RequestMessage in accordance with the priority of update.

The foreign agent or the home agent of the network domain to which thecorrespondent terminal currently belongs may maintain a plurality ofcare-of addresses for the mobile terminal and forward the packetdestined to a home address of the mobile terminal to each of theplurality of care-of addresses.

The foreign agent or the home agent of the network domain visited by thecorrespondent terminal may acquire the care-of address of the mobileterminal from the foreign agent or the home agent of the network domainfrom which the visiting correspondent terminal arrives.

The foreign agent or the home agent of the network domain to which thecorrespondent terminal currently belongs may monitor a volume of packetsdestined for the mobile terminal or packets from the mobile terminal soas to control update of the care-of address of the mobile terminal inaccordance with the monitored volume.

The foreign agent or the home agent may suspend update of the care-ofaddress when the correspondent terminal moves out of the network domain.

The foreign agent or the home agent may resume the suspended update ofthe care-of address when a predetermined condition is met.

The aforementioned objects can also be achieved by an agent apparatusfor a communication system in which a mobile terminal communicates witha correspondent terminal, operated as a foreign agent or a home agentfor a network domain to which the correspondent terminal belongs,comprising: receiver for receiving a Binding Update Message from a homeagent for the mobile terminal; and transmitter for forwarding a packetdestined for the mobile terminal to a current care-of address of themobile terminal designated in the Binding Update Message.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and further features of the present invention will beapparent from the following detailed description when read inconjunction with the accompanying drawings, in which:

FIG. 1 shows an example of a network to which route optimizationaccording to a first embodiment is applied;

FIG. 2 is an SDL diagram showing an operation of a foreign agent;

FIG. 3 shows an example of an attached terminal management tableprovided in the foreign agent with which a mobile terminal is connected;

FIG. 4 shows an example of a binding cache table provided in the foreignagent with which a correspondent terminal is connected;

FIG. 5 is an SDL diagram showing an operation of the correspondentterminal;

FIG. 6 is an SDL diagram showing an operation of the mobile terminal;

FIG. 7 is an SDL diagram showing an operation of a home agent;

FIG. 8 shows an example of a care-of address management table providedin the home agent for the mobile terminal;

FIG. 9 shows a sequence of care-of address registration and packettransmission;

FIG. 10 shows an example of a network to which the route optimizationaccording to a third embodiment is applied;

FIG. 11 shows an example of an extended Binding Request Messageaccording to the third embodiment;

FIG. 12 shows an example of a HA support table;

FIG. 13 is an SDL diagram showing an operation performed when the homeagent capable of interpreting the extended Binding Request Messagereceives the Binding Request Message;

FIG. 14 shows an example of a HA support table in which a subnet mask isintroduced;

FIG. 15 shows an example of a Binding Update Message used when the HAsupport table is dynamically set;

FIG. 16 shows an example of an internal table in which selected homeagents are registered;

FIG. 17 shows an example of an internal table in which grouping using asubnet mask is introduced;

FIG. 18 shows an example of a service setting table;

FIG. 19 is an SDL diagram showing an operation of a foreign agentaccording to an eighth embodiment receiving an IP packet;

FIG. 20 shows an example service setting table in which a subnet mask isintroduced;

FIG. 21 is an SDL diagram showing an operation for adjusting an intervalof transmission of a Binding Request Message from the foreign agent;

FIG. 22 shows a binding cache table of the foreign agent according tothe tenth embodiment;

FIG. 23 shows an example of a service setting table provided in theforeign agent according to an eleventh embodiment;

FIG. 24 shows an example of an extended Binding Update Message accordingto a twelfth embodiment;

FIG. 25 shows that the correspondent terminal of the network shown inFIG. 1 moves to a remote IP network;

FIG. 26 is an SDL diagram showing an operation of the foreign agentaccording to a thirteenth embodiment;

FIG. 27 is an SDL diagram showing an operation of the foreign agentaccording to the thirteenth embodiment;

FIG. 28 shows a sequence of operations performed by terminals and agentswhen the correspondent terminal moves to a remote IP network;

FIG. 29 shows an example of an extended Binding Acknowledge Messageaccording to the thirteenth embodiment;

FIG. 30 shows how a packet is transmitted according to the Mobile IPprotocol of the related art; and

FIG. 31 shows Route Optimization applied to the related-art Mobile IP.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

First Embodiment

FIG. 1 shows an example of a network to which the route optimizationaccording to a first embodiment is applied.

Referring to FIG. 1, an IP network (network domain) 1 is a home domainfor a mobile terminal 11. An IP network (network domain) 3 is adestination domain of the mobile terminal 11. An IP network (networkdomain) 4 is a home domain for a correspondent terminal 41. An IPnetwork (network domain) 5 is a destination domain of the correspondentterminal 41. Numeral 6 indicates an additional IP network (networkdomain). The Internet 2 connects the IP networks 1, 3-6 to each other.

The mobile terminal 11 has a permanent IP address in the home domain(home address). A base station 12 is connected to the home agent 13 soas to communicate with mobile terminals located in the IP network 1. Ahome agent (agent apparatus) 13 is located in the home domain to whichthe mobile terminal 11 belongs. In the illustrated example, the networkaddress of the IP network 1 is 10.75.51.0, the home address of themobile terminal 11 is 10.76.51.100 and the IP address of the home agent13 is 10.76.51.254.

A base station 31 is connected to the foreign agent 32 and communicateswith the mobile terminal 11 located in the IP network 3 and with thecorrespondent terminal 41 over a radio channel. A foreign agent (agentapparatus) 32 is located in the IP network 3. The network address of theIP network 3 is 10.76.52.0 and the IP address of the foreign agent 32 is10.76.52.254.

The correspondent terminal 41 communicates with the mobile terminal 11.The correspondent terminal 41 may be a land unit or a mobile unit. Abase station 42 is connected to a home agent 43 and communicates withterminals located in the IP network 4 over a radio channel. The homeagent (agent apparatus) 43 is located in the home domain to which thecorrespondent terminal 41 belongs. The network address of the IP network4 is 10.76.55.0, the home address of the correspondence terminal 41 is10.76.55.100 and the IP address of the home agent 43 is 10.76.55.254.

A base station 51 is connected to a foreign agent 52 and communicateswith terminals located in the IP network 5. The foreign agent (agentapparatus) 52 is located in the IP network 5. The network address of theIP network 5 is 10.76.53.0 and the IP address of the foreign agent 52 is10.76.53.254.

A base station 61 is connected to a foreign agent 62 and communicateswith terminals located in the IP network 6. The foreign agent (agentapparatus) 62 is located in the IP network 6 and has a care-of address.The network address of the IP network 6 is 10.76.54.0 and the IP addressof the foreign agent 62 is 10.76.54.254.

The home agent 13, the mobile terminal 11 and the correspondent terminal41 operate as entities that comply with the Mobile IP specification.

A description will now be given of the operation of the terminals andagents.

First, the operation of the foreign agents 32, 52 and 62 will bedescribed. FIG. 2 is a specification and description language (SDL)diagram showing the operation of the foreign agents 32, 52 and 62. FIG.3 shows an example of an attached terminal management table provided inthe foreign agent (32, 52, 62) attached to the mobile terminal 11. FIG.4 shows an example of a biding cache table provided in the foreign agent(32, 52, 62) attached to the correspondent terminal 41.

When the mobile terminal 11 moves to the IP network (3, 5, 6), themobile terminal 11 transmits a Registration Request Message destined forthe home agent 13 to the foreign agent (32, 52, 62) of the destinationIP network (3, 5, 6) via the base station (31, 51, 61). The RegistrationRequest Message includes the home address of the mobile terminal 11, theaddress of the home agent 13 and the care-of address acquired in the IPnetwork (3, 5, 6).

In step ST11, the foreign agent (32, 52, 62) receives the RegistrationRequest Message from the mobile terminal 11 located in the home IPnetwork (3, 5, 6) via the base station (31, 51, 61). In step ST12, theforeign agent (32, 52, 62) searches the attached terminal managementtable as shown in FIG. 3. In step ST13, the foreign agent (32, 52, 62)makes a determination as to whether the mobile terminal 11 is registeredin the attached terminal management table. If it is determined that themobile terminal 11 is registered in the attached terminal managementtable, registered information is updated bases on the RegistrationRequest Message in step ST14. If it is determined that the mobileterminal 11 is not registered in the attached terminal management table,the foreign agent (32, 52, 62) registers the mobile terminal 11 in theattached terminal management table in step ST15 based on theRegistration Request Message.

In step ST16, the foreign agent (32, 52, 62) forwards the RegistrationRequest Message to the home agent 13 for the mobile terminal 11.

In step ST21, the foreign agent (32, 52, 62) receiving an IP packet froma terminal in the Internet 2 or the home IP network (3, 5, 6) examinesthe header portion of the IP packet so as to determine whether the IPpacket is destined for an IP address in the home IP network (3, 5, 6).If is determined that the IP packet is destined for an IP address in thehome IP network (3, 5, 6), the foreign agent (32, 52, 62) decapsulatesthe IP packet in step ST23 and delivers the original IP packet to acorresponding terminal in the home IP network (3, 5, 6) in step ST24.

When it is determined that the IP packet is not destined for an IPaddress in the home IP network (3, 5, 6), the foreign agent (32, 52, 62)searches the binding cache table as shown in FIG. 4 (described later) instep ST25. In step ST26, the foreign agent (32, 52, 62) determineswhether the mobile terminal 11, to which the IP packet is addressed, isregistered in the binding cache table. If it is determined that themobile terminal 11 is registered in the binding cache table, the foreignagent (32, 52, 62) reads the care-of address for the mobile terminal 11from the binding cache table in step ST27. The foreign agent (32, 52,62) then encapsulates the IP packet and forwards, in step ST28, theencapsulated IP packet to the care-of address of the mobile terminal 11to which the original IP packet is addressed. If it is determined thatthe mobile terminal 11 is not registered in the binding cache table, theforeign agent (32, 52, 62) forwards the IP packet in step ST28 withoutencapsulation.

In step ST31, the foreign agent (32, 52, 62) receives a Binding UpdateMessage from the home agent 13 for the mobile terminal 11. In step ST32, the foreign agent (32, 52, 62) searches the binding cache table asshown in FIG. 4. In step ST33, the foreign agent (32, 52, 62) determineswhether the mobile terminal 11, specified in the Binding Update Message,is registered in the binding cache table. If it is determined that themobile terminal 11, specified in the Binding Update Message, isregistered in the binding cache table, the foreign agent (32, 52, 62)updates registered information in step ST34 based on the Binding UpdateMessage. If it is determined that the mobile terminal, specified in theBinding Update Message, is not registered in the binding cache table,the foreign agent (32, 52, 62) registers the mobile terminal 11 in thebinding cache table in step ST35 based on the Binding Update Message.The Binding Update Message includes the home address of the mobileterminal 11, the care-of address thereof, the address of the home agentand a timeout value of the Message. The binding cache table alsoincludes similar information registered therein.

A description will now be given of the operation of the correspondentterminal 41. FIG. 5 is an SDL diagram showing the operation of thecorrespondent terminal 41. In step ST41, the correspondent terminal 41receives the IP packet via the foreign agent (32, 52, 62) or the homeagent 43. The foreign agent (32, 52, 62) or the home agent 43 isresponsible for encapsulation and decapsulation so that thecorrespondent terminal 41 according to the first embodiment does nothave to handle encapsulation and decapsulation.

A description will now be given of the operation of the mobile terminal11. FIG. 6 is an SDL diagram showing the operation of the mobileterminal 11. In step ST51, the mobile terminal 11 receives aRegistration Acknowledge Message from the home agent 13 in response to aRegistration Request Message. In step ST61, the mobile terminal 11receives an agent advertisement from the foreign agent (32, 52, 62) ofthe IP network (3, 5, 6) in which the mobile terminal 11 is located. Instep ST62, the mobile terminal 11 acquires a care-of addresscorresponding to the agent advertisement. In step ST63, the mobileterminal 11 generates a Registration Request Message that includes thecare-of address of the mobile terminal 11 and transmits the RegistrationRequest Message to the home agent 13 via the foreign agent (32, 52, 62).In step ST71, the mobile terminal 11 receives the IP packet via theforeign agent (32, 52, 62) or the home agent 13.

FIG. 7 is an SDL diagram showing the operation of the home agent 13.FIG. 8 shows an example of a care-of address management table for themobile terminal 11 provided in the home agent 13.

Upon receipt of the Registration Request Message from the mobileterminal 11 in step ST81, the home agent 13 for the mobile terminal 11searches the care-of address management table as shown in FIG. 8 in stepST82. In step ST83, the home agent 13 determines whether the mobileterminal 11 is registered in the care-of address management table. If itis determined that the mobile terminal 11 is registered in the care-ofaddress management table, the home agent 13 updates informationregistered in the table based on the Registration Request Message instep ST84. If it is determined that the mobile terminal 11 is notregistered in the care-of address management table, the home agent 13registers the mobile terminal 11 in the care-of address management tablebased on the Registration Request Message in step ST85.

In step ST86, the home agent 13 transmits a Registration AcknowledgeMessage to the mobile terminal 11 indicating that registration iscomplete.

Upon receipt of an IP packet from a terminal in the Internet 2 or thehome IP network (3, 5, 6) in step ST91, the home agent 13 searches thecare-of address management table in step ST92. In step ST93, the homeagent 13 determines whether the mobile terminal 11, to which the IPpacket is addressed, is registered in the care-of address table.

If it is determined that the mobile terminal 11, to which the IP packetis addressed, is registered in the care-of address management table, thehome agent 13 reads the care-of address of the mobile terminal 11 fromthe care-of address in step ST94 so as to encapsulate the IP packet. Instep ST95, the home agent 13 transmits a Binding Update Message for themobile terminal 11, to which the IP packet is addressed, to a sender ofthat IP packet. In step ST96, the home agent 13 forwards theencapsulated IP packet to the mobile terminal 11. If it is determined instep ST93 that the mobile terminal 11 is not registered in the care-ofaddress management table, the home agent 13 determines that the mobileterminal 11 is located in the home domain. In step ST96, the home agent13 delivers the IP packet to the mobile terminal 11 via the base station12.

When receiving in step ST101 a Binding Request Message for the mobileterminal 11 from the foreign agent (32, 52, 62) of the domain to whichthe correspondent terminal 41 currently belongs, the home agent 13searches the care-of address management table as shown in FIG. 8 in stepST102. In step ST103, the home agent 13 transmits a Binding UpdateMessage for mobile terminal 11 to the contacting foreign agent (32, 52,62).

The home agent 43 may also be configured to perform a similar operationwith respect to the correspondent terminal 41.

A description will now be given of how a care-of address of the mobileterminal 11 is registered in the home agent 13 when the mobile terminal11 moves from the IP network 1 to the IP network 3, and how thecorrespondent terminal 41 sends a packet to the mobile terminal 11. FIG.9 shows a sequence of registration of the care-of address of the mobileterminal 11 and packet transmission. The base stations 12, 31, 42, 51and 61 only perform transparent transmission so that the operationthereof is omitted. Since the correspondent terminal 41 remainsstationary, it may be considered as a land unit.

A description will now be given of how the care-of address of the mobileterminal 11 is registered in the home agent 13 when the mobile terminal11 has moved from the IP network 1 to the IP network 3.

When the mobile terminal 11 has moved from the IP network 1 to the IPnetwork 3, the mobile terminal 11 acquires the care-of address throughthe Dynamic Host Configuration Protocol (DHCP). The mobile terminal 11registers the care-of address in the foreign agent 32 and the home agent13 by transmitting a Registration Request Message to the foreign agent.32. The Registration Request Message includes the care-of address of themobile terminal 11, the home address of the mobile terminal 11, theaddress of the home agent 13 for the mobile terminal 11.

The foreign agent 32 receiving the Registration Request Messagedetermines that the mobile terminal 11 is visiting the home IP network3. The foreign agent updates the attached terminal management table sothat the home address 10.76.51.100 of the mobile terminal 11 is mappedinto the address 10.76.51.254 of the home agent 13.

Further, the foreign agent 32 forwards the Registration Request Messageto the home agent 13 so that the new care-of address of the mobileterminal 11 is registered in the home agent 13 for the mobile terminal11.

Upon receipt of the Registration Request Message, the home agent 13updates the care-of address management table shown in FIG. 8 so that thecare-of address 10.76.52.254 included in the Registration RequestMessage is mapped into the home address 10.76.51.100 of the mobileterminal 11.

Before moving to the IP network 3, the mobile terminal 11 wasaccommodated in the home IP network 1 so that the mobile terminal 11 didnot have a care-of address. Correspondence between the home address ofthe mobile terminal 11 and the care-of address was not registered in thecare-of address management table. Thus, when the mobile terminal 11moves to the IP network 3, the care-of address 10.76.52.254 is deliveredto the home agent 13 for the first time. The home agent 13 registers thecare-of address of the mobile terminal 11 in the care-of addressmanagement table shown in FIG. 8 for the first time. The home agent 13transmits a Registration Acknowledge Message to the sender of theRegistration Request Message to indicate that the care-of address isregistered for the first time in the care-of address management table orthe care-of address management table is updated.

The foreign agent 32 receiving the Registration Acknowledge Messageforwards it to the mobile terminal 11 in order to inform the mobileterminal 11 that registration is complete.

When the registration of the mobile terminal 11 in the home agent 13 iscomplete, the information relating to the mobile terminal 11 isregistered as shown in FIG. 3 in the attached terminal management tableof the foreign agent 32. The information relating to the mobile terminal11 is also registered as shown in FIG. 8 in the care-of addressmanagement table of the home agent 13. The process of registrationdescribed above is in accordance with the specification of the Mobile IPprotocol.

A description will now be given of the routing between the correspondentterminal 41 and the mobile terminal 11.

The correspondent terminal 41 transmits an IP packet to the mobileterminal 11 without encapsulation. The IP packet is received by theforeign agent 52 of the IP network 5 to which the correspondent terminal41 currently belongs. If the address of the IP packet (home address10.76.51.100 of the mobile terminal 11) is not identified in the bindingcache table shown in FIG. 4, the foreign agent 52 transmits the IPpacket transparently to the destination.

The home agent 13 receiving the IP packet encapsulates the IP packetbased on the information registered for the mobile terminal 11 in thecare-of address management table generated in the registration process.The home agent 13 then tunnels the encapsulated packet to the care-ofaddress 10.76.52.254.

The IP packet tunneled by the home agent 13 arrives at the foreign agent32. The packet is decapsulated by the foreign agent 32 so that theoriginal IP packet arrives at the mobile terminal 11.

The packet follows a path according to the triangular routing since thebinding cache table of the foreign agent 52 does not include informationregistered for the mobile terminal 11.

When tunneling the IP packet, the home agent 13 sends a Binding UpdateMessage to the correspondent terminal 41. The foreign agent 52 receivesthe Binding Update Message so as to register the information relating tothe mobile terminal 11 in the binding cache table as shown in FIG. 4.Since the corresponding terminal 41 is not supplied with the BindingUpdate Message, it continues to send packets to the home address10.76.51.100 of the mobile terminal 11 without encapsulation.

After the information relating to the mobile terminal 11 is registeredin the binding cache table of the foreign agent 52, the foreign agent 52encapsulates IP packets from the correspondent terminal 41 based on theinformation registered in the binding cache table for the mobileterminal 11. The foreign agent 52 tunnels the encapsulated packet to thecare-of address 10.76.52.254 of the mobile terminal 11. Morespecifically, the foreign agent 52 reads the destination IP address10.76.51.100 from the header portion of the IP packet from thecorrespondent terminal 41 and searches for the registered informationbound to the home address identical to the destination IP address. Theforeign agent 52 determines that the care-of address 10.76.52.254 boundto the registered information is the care-of address of the mobileterminal 11.

Thus, the encapsulated IP packet arrives at the foreign agent 32,bypassing the home agent 13. In this way, the disadvantage withtriangular routing is resolved.

As described, according to the first embodiment, the Binding UpdateMessage from the home agent 13 is received by the foreign agent (32, 52,62) to which the correspondent terminal 41 is attached so that theforeign agent (32, 52, 62) encapsulates the IP packet addressed to themobile terminal 11 before transmission. Accordingly, the need forperiodical communication between the correspondent terminal 41 and thehome agent 13 for the mobile terminal 11 is eliminated, thus reducingthe power consumption of the correspondent terminal 41. Moreover, thetransmission bandwidth is efficiently used by reducing the volume ofdata transmitted from the correspondent terminal 41 to the foreign agent52 of the domain to which the correspondent terminal 41 belongs.

Second Embodiment

The foreign agent (32, 52, 62) according to a second embodimenttransmits a Binding Request Message to the home agent 13 before theexpiration of lifetime specified in the Binding Update Message, in orderto prompt the home agent 13 for transmission of a Binding UpdateMessage. The Binding Request Message complies with the Internet draft(draft-ietf-mobileip-optim-08.txt). In this case, the Binding RequestMessage includes the home address of the mobile terminal 11.

The home agent 13 receiving the Binding Request Message returns theBinding Update Message including the latest care-of addresscorresponding to the home address of the mobile terminal 11 to thesender of the Binding Request Message. The foreign agent (32, 52, 62)receiving the Binding Update Message updates the care-of address of themobile terminal 11 in the binding cache table based on the BindingUpdate Message.

The other aspects of the second embodiment are the same as thecorresponding aspects according to the first embodiment so that thedescription thereof is omitted.

As described, according to the second embodiment, it is ensured that theforeign agent (32, 52, 62) of the domain to which the correspondentterminal 41 belongs periodically sends a message to the home agent 13 inorder to acquire the latest care-of address of the mobile terminal 11.Thereby, route optimization is achieved without the periodicalcommunication between the correspondent terminal 41 and the home agent13. With this, packets are tunneled to the latest care-of address.Moreover, the volume of data transmitted from the correspondent terminal41 to the foreign agent (32, 52, 62) of the domain to which thecorrespondent terminal 41 belongs is reduced.

Third Embodiment

FIG. 10 shows an example of a network to which the route optimizationmethod according to a third embodiment is applied. Referring to FIG. 10,the IP network 1 is the home domain for mobile terminals 11-1 and 11-2.The mobile terminals 11-1 and 11-2 communicate with correspondentterminals 41-1 and 41-2, respectively. The IP network 4 is the homedomain for the correspondent terminals 41-1 and 41-2. The home addressof the correspondent terminal 41-1 is 10.76.55.100 and the home addressof the mobile terminal 11-1 which communicates with the correspondentterminal 41-1 is 10.76.51.100. The home address of the correspondentterminal 41-2 is 10.76.55.101 and the home address of the mobileterminal 11-2 which communicates with the correspondent terminal 41-2 is10.76.51.101.

As indicated in FIG. 10, the foreign agent 52 (32, 62) according to thethird embodiment combines a plurality of Binding Request Messages,requesting the current mobility binding of the mobile terminals 11-1 and11-2, into an extended Binding Request Message, when the correspondingterminals 41-1, 41-2 located in the home IP network 5 (3, 6)respectively communicate with the mobile terminals 11-1 and 11-2 thatbelong to the same home domain. The foreign agent 52 (32, 62) transmitsthe extended Binding Request Message to the home agent 13.

The foreign agent 52 (32, 62) according to the third embodiment isprovided with a HA support table in which home agents capable ofinterpreting an extended Binding Request Message are listed. The foreignagent 52 (32, 62) refers to the HA support table so as to send theextended Binding Request Message to a home agent only when the homeagent is capable of interpreting an extended Binding Request Message.Otherwise, the foreign agent 52, (32, 62) sends a normal (unextended)Binding Request Message.

FIG. 11 shows an example of an extended Binding Request Messageaccording to the third embodiment. As shown in FIG. 11, in addition tothe information included in the normal Binding Request Message, theextended Binding Request Message includes an extension type 101, whichis permanently set to “38”, a length of the extension 102, a vendoridentifier 103, which is a vendor-specific identifier, a vendorextension type 104, which is a vendor-specific extension type, and homeaddresses 105 and 106 of the mobile terminals for which the BindingUpdate Message is requested.

The message extension described above is in conformity with the criticalvendor extension format of the IETF Internet draft(draft-ietf-vender-ext-05. txt). According to this draft, when theforeign agent not capable of interpreting the extension receives themessage extended in accordance with the critical vendor extensionformat, the whole message is ignored. Accordingly, when the home agentreceives the extended Binding Request Message, the home agent doesnothing to the message.

FIG. 12 shows an example of a HA support table. As shown in FIG. 12, thetable defines the mapping between the IP address of the home agent 13for the mobile terminal and information indicating whether the homeagent is capable of interpreting the extended Binding Request Message.

A description will now be given of the operation according to the thirdembodiment.

When the correspondent terminals 41-1 and 41-2 located in the home IPnetwork 5 (3, 6) respectively communicate with the mobile terminals 11-1and 11-2 having the same home domain, the foreign agent 52 (32, 62)according to the third embodiment sends the extended Binding RequestMessage to the home agent 13 capable of interpreting the extendedBinding Request Message by referring to the HA support table. Theforeign agent 52 (32, 62) sends the normal (unextended) Binding RequestMessage to the home agent 13 not having the interpreting capability.

A description will now be given of the operation of the home agent 13capable of interpreting the extended Binding Request Message. FIG. 13 isan SDL diagram showing the operation of the home agent capable ofinterpreting the extended Binding Request Message when it received theextended Binding Request Message.

In steps ST101-ST103, the home agent 13 according to the thirdembodiment reads, from the care-of address management table, the latestcare-of address corresponding to the home address of the mobile terminalincluded in unextended portion of the received Binding Request Message.The home agent 13 then sends a Binding Update Message that includes thelatest care-of address to the foreign agent 52 (32, 62) originating theBinding Request Message.

Subsequently, in step ST111, the home agent 13 determines whether thereceived Binding Request Message includes an extended portion. If it isdetermined that the Message does not contain an extended portion, theprocess is terminated. If it is determined that the Message contains anextended portion, control is turned to step ST12 where the home agent 13reads, from the care-of address management table, the latest care-ofaddresses corresponding to the home addresses of the mobile terminalslisted in the extended portion. In step ST113, the home agent 13transmits the Binding Update Message that includes the latest care-ofaddresses to the sender of the Binding Request Message. The home agent13 repeats the steps ST111-ST113 until the home addresses of the mobileterminals listed in the extended portion are exhausted.

The other aspects of the operation are the same as corresponding aspectsaccording to the first embodiment so that the description thereof isomitted.

As described, according to the third embodiment, when the correspondentterminals 41-1 and 41-2 respectively communicate with the mobileterminals 11-1 and 11-2 having the same home domain, the foreign agent52 (32, 62) combines Binding Request Messages, requesting the currentmobility binding of the mobile terminals 11-1 and 11-2, into an extendedBinding Request Message. The foreign agent 52 (32, 62) then sends theextended Binding Request Message. In this way, the volume of datatransmitted from the foreign agent 52 (32, 62) to the home agent 13 isreduced. As a result, the switching load imposed on the routers and thenetwork load are reduced.

Fourth Embodiment

The foreign gent (32, 52, 62) according to a fourth embodimentintroduces a subnet mask in the HA support table so that the capabilityor the absence thereof of interpreting the extended Binding RequestMessage is specified for a set of home agents 13 included in the subnet.

FIG. 14 shows an example of a HA support table in which a subnet isintroduced. In the HA support table shown in FIG. 14, the home agents 13on the IP network belonging to the IP network having the net address10.76.0.0 are indicated as being capable of interpreting the extendedBinding Request Message with the exception of the agent having the IPaddress 10.76.51.254.

The other aspects of the operation are the same as the correspondingaspects according to the third embodiment so that the descriptionthereof is omitted.

As described, according to the fourth embodiment, a group of home agents13 in a given subnet are indicated as being or not being capable ofinterpreting an extended Binding Request Message. Accordingly, the HAsupport table is generated efficiently.

Fifth Embodiment

The foreign agent (32, 52, 62) according to a fourth embodimentdynamically configures a HA support table according to the thirdembodiment, in accordance with a notification from the home agent 13.

The home agent 13 for the mobile terminal 11 notifies the foreign agent(32, 52, 62) of the IP network (3, 5, 6) to which the correspondentterminal 41 belongs that the home agent 13 is capable of interpreting anextended Binding Request Message. The home agent 13 appends anextension, indicating that it has the capability, to a Binding UpdateMessage transmitted at an opportunity other than the opportunity torespond to a Binding Request Message.

FIG. 15 shows an example of a Binding Update Message for dynamicallysetting the HA support table. As shown in FIG. 15, in addition to theinformation included in a normal Binding Update Message, the BindingUpdate Message according to the fifth embodiment includes an extensiontype 111, which is permanently set to “134”, a length of the extension112, a vendor identifier 113, which is a vendor-specific identifier, anda vendor extension type 114, which is a vendor-specific extension type.

Upon receipt of the extended Binding Update Message, the foreign agent(32, 52, 62) rewrites the information registered in the HA support tablefor the home agent 13 originating the Binding Update Message.

The message extension described above is in conformity with the normalvendor extension format of the IETF Internet draft(draft-ietf-vender-ext-05.txt). According to the draft, when the foreignagent not capable of interpreting the extension receives a messageextended in accordance with the normal vendor extension format, theforeign agent should is requested to process the Message by ignoring theextension. In other words, when the foreign agent not capable ofinterpreting the extension receives a Binding Update Message having anextension, only the extension is ignored so that the remaining portionsare normally processed.

The other aspects of the fifth embodiment are the same as thecorresponding aspects according to the first embodiment so that thedescription thereof is omitted.

As described, according to the fifth embodiment, the foreign agent (32,52, 62) is allowed to dynamically set a HA support table in accordancewith a notification from the home agent 13. Accordingly, a networkmanager need not manually set the HA support table of the foreign agent(32, 52, 62). Indication as to whether the home agent 13 is capable ofinterpreting an extended Binding Request Message is automatically set inthe HA support table. An advantage of the fifth embodiment is that thesetting of the HA support table is simplified.

Sixth Embodiment

The foreign agent (32, 52, 62) according to a sixth embodiment accepts aBinding Update Message from the selected home agent 13.

Route Optimization fails when a care-of address is changed by a BindingUpdate Message with a malicious intent. When this failure occurs, apacket arrives at an undesired destination instead of arriving at aproper destination. It is thus preferable, from the standpoint ofsecurity, that only those Binding Update Messages from a trusted homeagent 13 be used. Accordingly, the foreign agent (32, 52, 62) accordingto the sixth embodiment accepts only those Binding Update Messages fromthe selected home agents 13.

The foreign agent (32, 52, 62) according to the sixth embodiment isprovided with an internal table in which the selected home agents 13 areregistered. The foreign agent (32, 52, 62) updates the care-of addressof the mobile terminal 11 only in accordance with the Binding UpdateMessages from the home agents 13 positively registered in the internaltable. Binding Update Messages from the other home agents 13 areignored.

FIG. 16 is an example of an internal table in which the selected homeagents 13 are specified. As shown in FIG. 16, each of the IP addressesof the home agents 13 listed is mapped into information indicatingwhether the Binding Update Message from the corresponding home agent isacceptable. In the case of the internal table of FIG. 16, the BindingUpdate Message from the home agent 13 having the IP address 10.76.51.254is accepted. The Binding Update Message from the other home agents 13 isnot accepted.

The other aspects of the operation according to the sixth embodiment arethe same as the corresponding aspects according to the third embodimentso that the description thereof is omitted.

As described, according to the sixth embodiment, only those BindingUpdate Message from the selected home agents 13 are accepted. Thus, itis ensured that only those Binding Update Messages from the trusted homeagents 13 are used, resulting in reinforcement of the security.

Seventh Embodiment

In a seventh embodiment, the home agents 13 registered in the internaltable provided in the foreign agent (32, 52, 62) according to the sixthembodiment are arranged into groups, using a subnet mask.

FIG. 17 shows an example of an internal table in which grouping using asubnet mask is introduced. In the case of the internal table shown inFIG. 17, only those Binding Update Messages from the home agents 13belonging to the subnet having the IP address 10.76.0.0 are acceptedwith an exception of the home agent 13 having the IP address10.76.55.254.

Thus, according to the seventh embodiment, the home agents 13 whoseBinding Update Message is accepted are listed using subnet grouping.With this, identification the selected home agents 13 assigned theauthority to issue an acceptable Binding Update Message is easilyimplemented.

Eighth Embodiment

The foreign agent (32 52, 62) according to an eighth embodimentencapsulates only those IP packets from selected correspondent terminals(41-1, 41-2) and tunnels the encapsulated packets.

The foreign agent (32, 52, 62) according to the eighth embodiment isprovided with a service setting table in which information indicatingwhether Route Optimization is executed for the correspondent terminal41-i (i=1, 2) located in the home IP network (3, 5, 6).

FIG. 18 shows an example of a service setting table. When the servicesetting table shown in FIG. 18 is provided in the network shown in FIG.10, the foreign agent 52 (32, 42) encapsulates IP packets form thecorrespondent terminal 41-1 having the home address 10.76.55.100 andtunnels the encapsulated packets. In contrast, the foreign agent 52forwards IP packets from the correspondent terminal 41-2 having the homeaddress 10.75.55.101 transparently.

A description will now be given of the operation according to the eighthembodiment.

FIG. 19 is an SDL diagram showing the operation of the foreign agent(32, 52, 62) according to the eighth embodiment.

Referring to the network shown in FIG. 10, the when the foreign agent(32, 52, 62) according to the eighth embodiment receives an OP packet instep ST21 via the Internet 2 or a correspondent terminal in the home IPnetwork (3, 5, 6), the foreign agent (32, 52, 62) examines the IP packetso as to determine whether the IP packet is destined for an address inthe home IP network (3, 5, 6). When it is determined that the IP packetis destined for an IP address in the home IP network (3, 5, 6), theforeign agent (32, 52, 62) decapsulates the IP packet in step ST23. Instep ST24, the foreign agent (32, 52, 62) delivers the IP packet to adestination terminal in the home IP network (3, 5, 6).

When it is determined that the received IP packet is not destined for anIP address in the home IP network (3, 5, 6), the foreign agent (32, 52,62) refers to the service setting table in step ST121 so as to determinein step ST122 whether the packet is to be processed with RouteOptimization.

When it is determined as a result of referring to the service settingtable that the packet is to be processed with Route Optimization, theforeign agent (32, 52, 62) determines in step ST123 whether the mobileterminal to which the IP packet is addressed is registered in thebinding cache table. When it is determined that the mobile terminal towhich the IP packet is addressed is registered in the binding cachetable, the foreign agent (32, 52, 62) encapsulates the IP packet in stepST125 and forwards the encapsulated IP packet to the care-of address ofthe mobile terminal in step ST126.

When it is determined that the packet is not to be processed with RouteOptimization as a result of referring to the service setting table, orwhen it is determined that the destination mobile terminal is notregistered in the binding cache table, the foreign agent (32, 52, 62)forwards the IP packet transparently in step ST126.

The other aspects of the eighth embodiment are the same as thecorresponding aspects according to the first embodiment so that thedescription thereof is omitted.

As described, according to the eighth embodiment, the foreign agent (32,52, 62) is configured to encapsulate only those IP packets from selectedcorrespondent terminals. A network manager is thus capable ofindividually specifying whether to execute Route Optimization. Datatolerant of delay, i.e. data not requiring real-time processing, may beprocessed without Route Optimization while data not tolerant of delay(such as VoIP data) are processed with Route Optimization, thus reducingthe load imposed on the foreign agent (32, 52, 62).

Ninth Embodiment

The foreign agent (32, 52, 62) according to a ninth embodimentintroduces a subnet mask in the service setting table. The need forRoute Optimization is inclusively specified for a group of correspondentterminals 41-i belonging to a subnet.

FIG. 20 shows an example of a service setting table in which a subnetmask is introduced. The service setting table shown in FIG. 20establishes that only those IP packets from the correspondent terminalsbelonging to the subnet 10.76.55.0 are processed with Route Optimizationwith the exception of the correspondent terminal having the IP address10.75.55.101.

The other aspects of the operation are the same as the correspondingaspects according to the eighth embodiment so that the descriptionthereof is omitted.

As described, according to the ninth embodiment, a subnet mask is usedto inclusively specify the need for Route Optimization for a group ofcorrespondent terminals belonging to the subnet. An advantage arisingfrom this arrangement is that the service setting table is generatedefficiently.

Tenth Embodiment

The foreign agent (32, 52, 62) according to a tenth embodiment adjuststhe interval of transmission of a Binding Request Message in accordancewith the frequency of change of the care-of address of the mobileterminal.

A description will now be given of the operation according to the tenthembodiment.

FIG. 21 is an SDL diagram showing the operation of the foreign agent(32, 52, 62) adjusting the interval of transmission of a Binding RequestMessage. FIG. 22 shows a binding cache table provided in the foreignagent (32, 52, 62) according to the tenth embodiment.

When the foreign agent (32, 52, 62) receives a Binding Update Message instep ST141, the foreign agent (32, 52, 62) searches the binding cachetable in step ST142 for the mobile terminal corresponding to the BindingUpdate Message.

In step ST143, the foreign agent (32, 52, 62) determines whether themobile terminal 11 is registered in the binding cache table and whetherthe care-of address included in the Binding Update Message is the sameas the care-of address in the binding cache table. When it is determinedthat the mobile terminal 11 is not registered in the binding cache tableor when it is determined that the care-of address included in theBinding Update Message is not the same as the care-of address in thebinding cache table, the foreign agent counts the frequency of change ofthe care-of address in step ST144. In step ST145, the care-of address inthe binding cache is updated so as to substitute therefor the care-ofaddress included in the Binding Update Message. in step ST145. When themobile terminal is not registered in the binding cache table, thefrequency of change of the care-of address is set to 0 and the mobileterminal is registered in the binding cache table.

As shown in FIG. 22, the frequency of change of the care-of address andthe priority of update are registered against each of the mobileterminal.

In step ST146, the foreign agent (32, 52, 62) computes the priority ofupdate in accordance with the frequency of change of the care-ofaddress.

When it is determined that the mobile terminal is registered in thebinding cache table and that the care-of address included in the BindingUpdate Message is the same as the care-of address in the binding cachetable, the process is terminated since there is no need to change thecare-of address.

In step ST131, when an internal timer of the foreign agent (32, 52, 62)gives a notice of timeout indicating that it is time to transmit aBinding Request Message, the foreign agent (32, 52, 62) searches thebinding cache table for the care-of address corresponding to the timerin step ST132. In step ST133, the foreign agent (32, 52, 62) transmits aBinding Request Message. In step ST134, the foreign agent (32, 52, 62)resets the frequency of change of the care-of address to 0 and computesa timer value in accordance with the priority of update. In step ST135,the foreign agent (32, 52, 62) starts the timer in order to reminditself of the occasion to transmit the next Binding Request Message.

The other aspects of the operation according to the tenth embodiment arethe same as the corresponding aspects according to the first embodimentso that the description thereof is omitted.

As described, according to the tenth embodiment, the foreign agent (32,52, 62) adjusts the interval of transmission of a Binding RequestMessage in accordance with the frequency of change of the care-ofaddress. Accordingly, exchange of Binding Request Messages and BindingUpdate Messages between the foreign agent (32, 52, 62) and the homeagent 13 is made to occur at an appropriate frequency. Thereby, thevolume of unnecessary messages exchanged is reduced, so that the volumeof traffic on the network is reduced.

Eleventh Embodiment

The foreign agent (32, 52, 62) according to an eleventh embodiment setsan initial value of the priority of update for each mobile terminal11-i.

FIG. 23 shows an example of a service setting table provided in theforeign agent (32, 52, 62) according to the eleventh embodiment. In theservice setting table shown in FIG. 23, the initial value of priority ofupdate for the mobile terminal 11-1 is set to 5, and the initial valueof priority of update for the mobile terminal 11-2 is set to 3. Thenetwork manager is responsible for setting the initial values.

A description will now be given of the operation according to theeleventh embodiment.

The foreign agent (32, 52, 62) of the IP network (3, 5, 6) to which thecorrespondent terminal currently belongs reads the initial value ofpriority of update for the mobile terminal 11-i and computes' a timervalue in accordance with the initial value of priority of update. Theforeign agent (32, 52, 62) then starts a timer for timing thetransmission of next Binding Request Message to the mobile terminal11-i.

The other aspects of the operation according to the eleventh embodimentare the same as the corresponding aspects according to the tenthembodiment so that the description thereof is omitted.

When the initial values of priority of update are set as shown in FIG.23, the Binding Request Message for the mobile terminal 11-1 having theIP address 10.76.51.100 is transmitted more frequently than the BindingRequest Message for the mobile terminal 11-2 having the IP address10.76.51.101, given that the lifetime of the Binding Update Message isunchanged.

As described, according to the eleventh embodiment, the foreign agent(32, 52, 62) sets the initial value of priority of update for eachmobile terminal 11-i. With this, it is ensured that the varying qualityof Route Optimization service is provided for a plurality of mobileterminals 11-i.

Twelfth Embodiment

The foreign agent (32, 52, 62) according to a twelfth embodimentregisters a plurality of care-of addresses for a mobile terminal in abinding cache table so that IP packets destined for the home address ofthe mobile terminal are tunneled to each of the plurality of care-ofaddresses.

When the mobile terminal is located at a boundary between a plurality ofIP networks, handover may take place frequently. In this situation, themobile terminal frequently notifies the home agent 13 of a change in thecare-of address, increasing the network load and power consumption ofthe mobile terminal. The Mobile IP protocol allows the mobile terminalat a boundary between a plurality of IP networks to register a pluralityof care-of addresses in order to eliminate the necessity for the mobileterminal to frequently change the care-of address. In triangularrouting, the home agent 13 in which the plurality of care-of addressesare registered forwards the IP packets destined for the mobile terminalto each of the plurality of care-of addresses.

However, since IP packets destined for the mobile terminal are notrouted via the home agent according to the related-art RouteOptimization, IP packets are forwarded to only one of the plurality ofcare-of addresses.

The foreign agent (32, 52, 62) according to the twelfth embodimentregisters, in the binding cache table, the plurality of care-ofaddresses registered in the home agent 13 so that the routes to thecare-of addresses are optimized.

A description will now be given of the operation according to thetwelfth embodiment.

FIG. 24 shows an example of an extended Binding Update Message accordingto the twelfth embodiment.

When the home agent 13 in which a plurality of care-of addresses areregistered for a given mobile terminal 11 receives a Binding RequestMessage for the mobile terminal 11, the home agent 13 sends an extendedBinding Update Message as shown in FIG. 24 to the foreign agent (32, 52,62) originating the Binding Request Message. The extended portion of theBinding Update Message contains second and subsequent care-of addresses.

The foreign agent (32, 52, 62) receiving the extended Binding UpdateMessage reads a first care-of address from the unextended portion andreads additional care-of addresses from the extended portion. Theforeign agent (32, 52, 62) then registers the care-of addresses thusread in the binding cache table in relation to the target mobileterminal 11.

The foreign agent (32, 52, 62) receiving an IP packet destined for thehome address of the mobile terminal 11 tunnels the IP packet to each ofthe plurality of care-of addresses corresponding to the home address.

The other aspects of the twelfth embodiment are the same as thecorresponding aspects of the first embodiment so that the descriptionthereof is omitted.

As described, according to the twelfth embodiment, the foreign agent(32, 52, 62) registers a plurality of care-of addresses for the mobileterminal 11 in the binding cache table, as the situation demands. IPpackets destined for the home address of the mobile terminal 11 aretunneled to each of the plurality of care-of addresses. Accordingly,even when the care-of address of the mobile terminal 11 located at aboundary between network domains changes frequently, it is ensured thatIP packets are properly delivered to the mobile terminal without goingthrough a process of address registration every time the care-of addressis changed.

Thirteenth Embodiment

When a correspondent terminal visits an IP network (3, 5, 6), theforeign agent (32, 52, 62) according to a thirteenth embodiment acquiresinformation registered in a binding cache table provided in the remoteforeign agent (32, 52, 62) belonging to the remote IP network (3, 5, 6)from which the visiting correspondent terminal arrived. The informationthus acquired is registered in the binding cache table of the acquiringforeign agent (32, 52, 62).

In the related-art Route Optimization, when a correspondent terminaloriginating an IP packet moves across IP networks, IP packets form thecorrespondent terminal to a target mobile terminal are temporarilyrouted via the home agent of the target mobile terminal until theforeign agent of the visited IP network receives a Binding UpdateMessage, since the foreign agent does not have information relating tothe correspondent terminal registered in the binding cache table.Consequently, the same operation performed according to a sequence ofFIG. 9 when the correspondent terminal transmits an IP packet for thefirst time is repeated whenever the correspondent terminal moves acrossIP networks. Thus, triangular routing occurs temporarily.

The foreign agent (32, 52, 62) according to the thirteenth embodimentresolves the problem of temporary triangular routing occurring when thecorrespondent terminal moves across IP networks, by allowing the foreignagent (32, 52, 62) of the visited IP network to acquire registeredinformation from the binding cache table of the remote foreign agent(32, 52, 62) of the remote IP network from which the visitingcorrespondent terminal arrived.

A description will now be given of the operation according to thethirteenth embodiment.

FIG. 25 shows that the correspondent terminal 41 of the network shown inFIG. 1 moves to the remote IP network 6. FIGS. 26 and 27 are SDLdiagrams showing the operation of the foreign agent (32, 52, 62)according to the thirteenth embodiment.

When the foreign agent 62 (32, 52) according to the thirteenthembodiment receives a Registration Request Message in step ST11, controlis successively turned to steps ST12, ST13, ST14 and ST15. In stepST151, a determination is made as to whether the Registration RequestMessage contains an extended portion (described later). If it isdetermined that the Registration Request Message contains an extendedportion, control is turned to step ST152, where the foreign agent 62(32, 52) sends a Binding Update Message informing a foreign agent 52(32, 62), to which the visiting correspondent terminal 41, of thecare-of address of the visiting correspondent terminal 41 originatingthe Registration Request Message. Subsequently, in step ST16, theforeign agent (32, 52, 62) forwards the Registration Request Message tothe home agent 43 for the correspondent terminal 41 originating theRegistration Request Message (or the home agent 13 for the mobileterminal).

When the foreign agent (32, 52, 62) according to the thirteenthembodiment receives an IP packet in step ST21 and when it is determinedin step ST22 that the IP packet is destined for an IP address in thehome IP network (3, 5, 6), the foreign agent decapsulates the IP packet.

In step ST161, the foreign agent (32, 52, 62) searches the attachedterminal management table for a terminal having the home addressidentical to the destination address of the IP packet. In step ST162, adetermination is made as to whether there is such a terminal. If it isdetermined that there is a terminal having the home address identical tothe destination address of the IP packet, it is determined that thedestination terminal is located in the home IP network (3, 5, 6). Instep ST24, the IP packet is delivered to that terminal.

If it is determined that the attached terminal management table does notlist a terminal having the home address identical to the destinationaddress of the IP packet, it is determined that the destination terminalis not located in the home IP network (3, 5, 6) of the foreign agent(32, 52, 62). In a subsequent step ST163, the foreign agent (32, 52, 62)then searches the binding cache table so as to determine in step ST164whether the packet destination terminal (mobile terminal 11) isregistered in the binding cache table. If it is determined that themobile terminal 11 to which the IP packet is addressed is registered inthe binding cache table, the foreign agent (32, 52, 62) decapsulates theIP packet in step ST165. In step ST166, the foreign agent (32, 52, 62)forwards the encapsulated IP packet to the current care-of address ofthe mobile terminal 11. In step ST167, an alarm is given indicating thatthe destination mobile terminal 11 is not attached to the IP network,(3, 5, 6) of the foreign agent (32, 52, 62). If it is determined thatthe destination mobile terminal 11 is not registered in the bindingcache table, the foreign agent (32, 52, 62) discards the IP packet.

When the foreign agent (32, 52, 62) receives a Binding Update Message instep ST131, control is successively turned to steps ST33, ST34 or ST35.In a subsequent step ST171, a determination is made as to whether theBinding Update Message is from another foreign agent (32, 52, 62). If itis determined that the Binding Update Message is from another foreignagent (32, 52, 62), a Binding Acknowledge Message containing thecontents of the local binding cache table is sent to the foreign agent(32, 52, 62) originating the Binding Update Message in step ST172.

When the foreign agent (32, 52, 62) receives the Binding AcknowledgeMessage in step ST181, the foreign agent (32, 52, 62) generates, in stepST182, a binding cache table based on the contents of the BindingAcknowledge Message or appends the contents to the existing cache table.

A description will now be given of the operation performed by terminalsand agents when the correspondent terminal 41 moves from the IP network5 to the IP network 6. FIG. 28 shows a sequence of operations performedby terminals and agents when the correspondent terminal 41 moves to aremote IP network 6. The base station (12, 31, 42, 51, 61) merelyperforms transparent transmission so that the description of theoperation thereof is omitted.

As shown in FIG. 25, upon moving from the IP network 5 to the IP network6, the correspondent terminal 41 acquires a care-of address via, forexample, an agent advertisement from the foreign agent 62.

The correspondent terminal 41 acquiring the care-of address notifies thehome gent 43 of the care-of address thus acquired and sends aRegistration Request Message to the foreign agent 62 of the IP network 6to which the correspondent terminal 41 currently belongs.

The Registration Request Message is an extended version including aninstruction for causing the foreign agent 62 to notify the foreign agent52 of the care-of address of the correspondent terminal. This extensionis in accordance with the IETF Internet draft(draft-ietf.mobileip-optim-08.txt). The extended Registration RequestMessage includes the old care-of address and the new care-of address ofthe correspondent terminal 41.

When receiving the Registration Request Message 6, the foreign agent 62generates a Binding Update Message for notifying the foreign agent 52 ofthe care-of address of the correspondent terminal 41 and sends thegenerated message to the foreign agent 52.

The foreign agent 52 receiving the Binding Update Message generates abinding cache table in which the old care-of address of thecorrespondent terminal 41 is mapped into the new care-of address. Theforeign agent 52 also sends a Binding Acknowledge Message to the foreignagent 62. Thereafter, when the foreign agent 52 receives an IP packetdestined for the old care-of address of the correspondent terminal 41,the foreign agent 52 tunnels the IP packet to the new care-of address.

The Binding Acknowledge Message transmitted from the foreign agent 52 isan extended version formed to ensure that the information registered inthe binding cache table provided in the foreign agent 52 is delivered tothe foreign agent 62. FIG. 29 shows an example of an extended BindingAcknowledge Message according to the thirteenth embodiment. As shown inFIG. 29, the extended portion of the Binding Acknowledge Messageincludes the home address of the mobile terminal 11 registered in thebinding cache table, the care-of address of the mobile terminal 11 andthe address of the home agent of the mobile terminal 11.

The foreign agent 62 receiving the extended Binding Acknowledgegenerates a binding cache table for the mobile terminal 11 based on thecontents of the extended portion of the Binding Acknowledge Message. Theforeign agent 62 sends the Binding Request Message to the home agent 13in order to inform the home agent 13 of the new care-of address of thecorrespondent terminal 41. Thereby, an IP tunnel 23 extending from theforeign agent 62 to the foreign agent 32 is formed.

The other aspects of the operation according to the thirteenthembodiment are the same as the corresponding aspects according to thefirst embodiment so that the description thereof is omitted.

As described, according to the thirteenth embodiment, when thecorrespondent terminal 41 moves across IP networks, the foreign agent(32, 52, 62) of the destination IP network (3, 5, 6) acquires theinformation registered in the binding cache table provided in theforeign agent (32, 52, 62) of the source IP network (3, 5, 6) so as toregister the acquired information in the local binding cache table. Withthis, a temporary triangular routing is prevented from occurring so thatRoute Optimization is properly maintained.

Fourteenth Embodiment

The foreign agent (32, 52, 62) according to the fourteenth embodimentmonitors the volume of IP packets exchanged between the correspondentterminal 41 belonging to the home IP network (3, 5, 6) and the mobileterminal 11. Depending on the volume monitored, the foreign agent (32,52, 62) refrains from updating the care-of address registered in thebinding cache table.

In accordance with the first through thirteenth embodiments describedabove, the foreign agent (32, 52, 62) periodically updates the care-ofaddress of the mobile terminal 11 once it is registered in the bindingcache table. An update occurs even when the correspondent terminal 41has not transmitted any IP packets. As a result, the care-of address ofthe mobile terminal 11 less frequently receiving an IP packet is alsoperiodically updated.

The foreign agent (32, 52, 62) according to the fourth embodimentrefrains from updating the care-of address of the mobile terminal 11less likely to receive an IP packet.

A description will now be given of the operation according to thefourteenth embodiment.

The foreign agent (32, 52, 62) of the IP network (3, 5, 6) to which thecorrespondent terminal 41 currently belongs reads a destination IPaddress and an origination IP address specified in a received packet soas to compute a volume of traffic between the mobile terminal 11 and thecorrespondent terminal 41.

When the traffic of IP packets between the mobile terminal 11 and thecorrespondent terminal 41 is absent for a predetermined period of time,the foreign agent (32, 52, 62) suspends the transmission of a BindingRequest Message and update of care-of address in the binding cachetable.

The other aspects of the operation according to the fourteenthembodiment are the same as the corresponding aspects according to thefirst embodiment so that the description thereof is omitted.

As described, according to the fourteenth embodiment, the foreign agent(32, 52, 62) monitors the volume of traffic of IP packets from themobile terminal 11 and IP packets destined for the mobile terminal 11.The foreign agent (32, 52, 62) refrains from updating the care-ofaddress of the mobile terminal 11 in the binding cache table dependingon the monitored volume. Accordingly, the volume of packets forexchanging messages between the foreign agent (32, 52, 62) and the homeagent 13 is reduced.

Fifteenth Embodiment

The foreign agent (32, 52, 62) according to the fifteen embodimentsuspends the update of care-of address registered in the binding cachetable when the correspondent terminal 41 moves out of the local IPnetwork (3, 5, 6).

A description will now be given of the operation according to thefifteenth embodiment.

Referring to the network of FIG. 25, when the correspondent terminal 41moved from the IP network 5 accommodating the foreign agent 52 to the IPnetwork 6 accommodating the foreign agent 62, a Binding Update Messageis transmitted from the foreign agent 62 to the foreign agent 52, asshown in FIG. 28.

With this, the foreign agent 52 is informed that the correspondentterminal 41 has left the IP network 5. Even when the Binding UpdateMessage is not sent to the foreign agent 52, the foreign agent 52determines that the correspondent terminal 41 left the IP home network 5when the care-of address for the correspondent terminal 41 is no longervalid.

After the correspondent terminal 41 left the local IP network 5, it isnot necessary for the foreign agent 52 to tunnel an IP packet from thecorrespondent terminal 41 to the care-of address of the mobile terminal11. Therefore, the foreign agent 52 suspends the update of care-ofaddress in the binding cache table when it is determined that thecorrespondent terminal 41 left the local IP network 5.

As described, according to the fifteenth embodiment, when thecorrespondent terminal moves across IP networks, the foreign agent (32,52, 62) of the source IP network (3, 5, 6) does not update the care-ofaddress registered in the binding cache table. Accordingly, the volumeof packets for exchanging messages between the foreign agent (32, 52,62) and the home agent 13 is reduced.

Sixteenth Embodiment

The foreign agent (32, 52, 62) according to the sixteenth embodimentresumes the update of care-of address when a certain condition is met.

A description will now be given of the operation according to thesixteenth embodiment.

The foreign agent (32, 52, 62) resumes the update of care-of addresswhen it receives a Binding Update Message for the care-of address forwhich the update was suspended in accordance with the fourteenth andfifteenth embodiments. Alternatively, the update of care-of address maybe resumed when tunneling transmission to the care-of address occurs.

Thus, according to the sixteenth embodiment, the update of care-ofaddress is resumed when a certain condition is met. Accordingly, theupdate of care-of address is performed at an appropriate frequencyadapted for the purpose. With this, the volume of packets for exchangingmessages between the foreign agent (32, 52, 62) and the home agent 13 isreduced.

In the description given above, the function of the foreign agent (32,52, 62) and the function of the home agent (13, 43) are assumed to beseparate from each other. Alternatively, the home agent (13, 43) may beprovided with the function of a foreign agent for terminals notbelonging to the home agent (13, 43).

The present invention is not limited to the above-described embodiments,and variations and modifications may be made without departing from thescope of the present invention.

1. A route optimization method for a communication system that allowscommunication between a mobile terminal and a correspondent terminal,for optimizing a route for communication between the correspondentterminal and the mobile terminal when the mobile terminal moves acrossnetwork domains, comprising the steps of: causing a foreign agent or ahome agent of a network domain to which the correspondent terminalcurrently belongs to receive a Binding Update Message from a home agentfor the mobile terminal and to forward a packet destined for the mobileterminal to a care-of address of the mobile terminal specified in theBinding Update Message.
 2. The route optimization method according toclaim 1, further comprising the steps of: causing the foreign agent orthe home agent of the network domain to which the correspondent terminalcurrently belongs to repeat transmission of a Binding Request Message tothe home agent for the mobile terminal in order to acquire the latestcare-of address; and causing the home agent for the mobile terminal totransmit, in response to the Binding Request Message, a BindingAcknowledge Message containing the latest care-of address of the mobileterminal to the foreign agent or the home agent of the network domain towhich the correspondent terminal currently belongs.
 3. The routeoptimization method according to claim 2, wherein the foreign agent orthe home agent of the network domain to which the correspondent terminalcurrently belongs accepts only the Binding Update Message from theselected home agents.
 4. The route optimization method according toclaim 3, wherein the foreign agent or the home agent of the networkdomain to which the corresponding terminal belongs uses a subnet mask tomaintain a list of home agents from which Binding Update Message isacceptable.
 5. The route optimization method according to claim 2,wherein the foreign agent or the home agent of the network domain towhich the correspondent terminal currently belongs controls an intervalof transmission of the Binding Request Message in accordance with afrequency of change of the core-of address.
 6. The route optimizationmethod according to claim 5, wherein the foreign agent or the home agentof the network domain to which the correspondent terminal currentlybelongs sets an initial value of priority of update for each mobileterminal, computes the priority of update in care-of address, and setsthe interval of transmission of the Binding Request Message inaccordance with the priority of update.
 7. The route optimization methodaccording to claim 2, wherein the foreign agent or 5 the home agent ofthe network domain to which the correspondent terminal currently belongsmonitors a volume of packets destined for the mobile terminal or packetsfrom the mobile terminal so as to control update of the care-of addressof the mobile terminal in accordance with the monitored volume.
 8. Theroute optimization method according to claim 2, wherein the foreignagent or the home agent suspends update of the care-of address when thecorrespondent terminal moves out of the network domain.
 9. The routeoptimization method according to claim 8, wherein the foreign agent orthe home agent resumes the suspended update of the care-of address whena predetermined condition is met.
 10. The route optimization methodaccording to claim 1, further comprising the steps of causing theforeign agent or the home agent of the network domain to which thecorrespondent terminal belongs to combine a plurality of Binding RequestMessages for obtaining the care-of addresses of, a plurality of mobileterminals into an extended Binding Request Message and to send theextended Binding Request Message, when the plurality of mobile terminalsare coupled to the same home agent.
 11. The route optimization methodaccording to claim 10, wherein the foreign agent or the borne agent ofthe network domain to which the correspondent terminal currently belongsuses subnet masking to maintain a list of home agents capable ofinterpreting an extended Binding Request Message, so as to send theextended Binding Request Message for the plurality of mobile terminalsto the home agent capable of interpretation and send the Binding RequestMessage for each mobile terminal to the other home agents.
 12. The routeoptimization method according to claim 10, the home agent for the mobileterminal notifies the foreign agent or the home agent of the networkdomain to which the correspondent terminal currently belongs whether thehome agent is capable of interpreting the extended Binding RequestMessage, and the foreign agent or the home agent of the network domainto which the correspondent terminal currently belongs dynamicallydetermines whether the home agent for the mobile terminal is capable ofinterpreting the extended Binding Request Message, based on thenotification, so that the foreign agent or the home agent sends theextended Binding Request Message for the plurality of mobile terminalsto the home agent capable of interpreting the extended Binding RequestMessage and sends the Binding Request Message for each mobile terminalto the home agent.
 13. The route optimization method according to claim1, wherein the foreign agent or the home agent forwards only the packetfrom the correspondent terminal to the mobile terminal.
 14. The routeoptimization method according to claim 13, wherein the foreign agent orthe home agent uses a subnet mask to designate a group of correspondentterminals with respect to route optimization.
 15. The route optimizationmethod according to claim 1, wherein the foreign agent or the home agentof the network domain to which the correspondent terminal currentlybelongs maintains a plurality of care-of addresses for the mobileterminal and forwards the packet destined to a home address of themobile terminal to each of the plurality of care-of addresses.
 16. Theroute optimization method according to claim 1, wherein the foreignagent or the home agent of the network domain visited by thecorrespondent terminal acquires the care-of address of the mobileterminal from the foreign agent or the home agent of the network domainfrom which the visiting correspondent terminal arrives.
 17. An agentapparatus for a communication system in which a mobile terminalcommunicates with a correspondent terminal, operated as a foreign agentor a home agent for a network domain to which the correspondent terminalbelongs, comprising: a receiver for receiving a Binding Update Messagefrom a home agent for the mobile terminal; and a transmitter forforwarding a packet destined for the mobile terminal to a currentcare-of address of the mobile terminal designated in the Binding UpdateMessage.